During the operation of high temperature articles, such as components of gas turbine engines, highly corrosive and oxidizing conditions can be experienced by exposed article surfaces. Therefore, development of the gas turbine art has included development of a variety of coatings resistant to such adverse conditions. Such known coatings include commercially available forms of Codep coating aluminiding, examples of which are included in such U.S. patents as U.S. Pat. Nos. 3,598,638--Levine (issued Aug. 10, 1971) and 3,667,985--Levine et al (issued Jun. 6, 1972). Associated with general aluminiding is a localized aluminiding through a patch-type coating such as is described in U.S. Pat. No. 4,004,047--Grisik, issued Jan. 18, 1977.
Other forms of high temperature coatings used for environmental protection include combinations of metals selected from the platinum group of metals, particularly Pt, Rh and Pd, along with aluminiding. Forms of this combination of coatings are described in U.S. Pat. Nos. 3,819,338--Bungardt et al (issued Jun. 25, 1974) and 3,979,273--Panzera et al (issued Sep. 7, 1976).
The disclosures of each of the above identified patents hereby are incorporated herein by reference.
Coatings of various types, including an overall platinum aluminide coating, have been reported and used as protective coatings for high temperature operating gas turbine engine components such as a high pressure turbine blade (HPTB). However, certain problems have been recognized during manufacture and/or use of such articles. For example, during the service life of a typical HPTB, several partial and at least one full repair generally will be required to extend the useful life of such a component, which originally is relatively expensive to manufacture. Complicating such later repairs can be the application, in original manufacture of the article, of an environmental protective coating, generally referred to as a thermal barrier coating, and based on ceramic type materials such as zirconia, generally stabilized with such materials as yttria. Being ceramic base, such coating has a greater tendency than would a metal base coating to be brittle and to spall if processed subsequently, such as in repair, after initial coating. Therefore, repair of discrete surface areas of a thermal barrier coated article is more difficult. In all cases, a reliable spot-type or discrete surface coating is needed for the repair or to enhance the environmental resistance of localized discrete selected surfaces of a high temperature operating article to which has been applied a surface environmental protective coating.